1. Field of the Invention
The present invention relates to an optical transmitting assembly used in high-speed optical communication (for example, 10 Gbps) that provides a temperature control function within a small sized package.
2. Related Prior Art
The optical communication strongly requests an optical transceiver applicable to the high-speed transmission with a miniaturized package. For example, a multi-source agreement (MSA) has been standardized for the miniaturized optical transceiver with a pluggable function at the transmitting speed of 10 Gbps. This standard rules to provide an optical receptacle capable of mating with a dual LC-typeM optical connector. The pitch between two lines is 6.25 mm. The height and the width of the pluggable transceiver are set to 18.35 mm and 8.5 mm, respectively, because these transceivers are mounted in the motherboard with high density. Therefore, such optical transceiver with a narrower width, i.e., with a narrower cross section, requires the miniaturized optical transmitting assembly and the miniaturized optical receiving assembly to be installed within such small sized optical transceiver.
On the other hand, as the transmission speed increases, the power consumption of semiconductor devices also increases to operate in stable for the high-speed signal. Moreover, to transmit the optical signal in a long distance at the high-speed, the semiconductor laser diode (herein after denoted as LD) must satisfy a severe chirping characteristic. The direct modulation for the LD can not satisfy such high quality specification,
One type of an optical modulator, so called the EA (Electro-Absorption) modulator, is well known, which utilizes the dependence of the absorption co-efficient of the semiconductor material on the electric field. An EA-DFB device that monolithically integrates the EA modulator with the DFB laser diode is also well known in the field. To operate this EA-DFB device in stable and in precise its output wavelength and output optical power requires the temperature of the device in stable. Accordingly, the thermoelectric cooler, typically the Peltier cooler, is necessary to install within the optical assembly. However, it will be necessary to enlarge the thermoelectric cooler to obtain the temperature controlled with good quality. Accordingly, the optical transceiver for the application of the high speed and the long distance requires the optical assembly that installs such enlarged thermoelectric cooler and is to be installed within the transceiver with the specified dimensions.
To obtain small-sized optical assembly, it will be necessary to have the thermoelectric cooler small, but it will restrict the capacity of the cooling of the thermoelectric cooler. In general, the thermoelectric cooler has an upper plate made of insulating material, typically aluminum nitride (AlN). It will be preferable to configure a carrier, on which the EA-DFB device is mounted, made of metal whose thermal expansion coefficient matches with that of those insulating material to prevent the thermal stress when these two material are in contact to each other. On the other hand, the carrier is preferably made of material with good thermal conductivity to reduce the thermal resistance between the laser diode, which becomes a heat source, and the upper plate, which becomes the cooling surface, accordingly, to relief the capacity of the cooling of the thermoelectric cooler.
From the aforementioned viewpoint, the carrier is typically made of CuW. However, the CuW has relatively large heat capacity. That is, assuming that the total heat capacity of materials installed on the thermoelectric cooler is C [J/C], and the thermoelectric cooler is booted under the ambient temperature 75° C., it is necessary to transport the heat Qt [W]=C×*(75−25)/30=1.67×C by the thermoelectric cooler to lower the temperature thereof to 25° C. Therefore, to install members having large heat capacity such as CuW disturbs the lightening of the capability of the thermoelectric cooler.
In the case that the EA-DFB device modulates light, it is necessary to carry the modulation signal over a few GHz to the EA-DFB device without degrading the quality thereof, accordingly, the transmission line carrying the signal is necessary to terminate immediate the EA-DFB device with a resistor having predefined resistance. Thus arranging the termination resistor can carry the signal to the EA device with good quality and without undesired reflection of the high frequency signal by matching the transmission impedance.
A typical EA-DFB device is necessary to be supplied, for the bias current, 100 mA at most to emit the DFB portion thereof. On the other hand, the forward voltage of the DFB portion becomes about 1.5 V, accordingly, the EA-DFB device generates heat of about 0.15 W only in the DFB portion. Moreover, an average bias voltage for the EA portion, which is a sum of the bias voltage and a half of the modulation amplitude, becomes about 1.5 V Therefore, assuming the value of the termination resistor is 50Ω, the termination resistor generates heat of 0.045 W. Moreover, the EA portion is known that it generally generates the current, due to the photo absorption, of about 15 mA, which equivalently generates heat of 0.015 W Thus, when using the EA-DFB device, much heat is generated compared with the case that the DFB-LD is directly modulated, which brings further subjects for miniaturization of the thermoelectric cooler.
Furthermore, in the EA-DFB device, in particular, the EA portion and the is DFB portion are integrated to each other, the high frequency signal applied to the EA portion has leaked to the DFB device that is inherently driven by the DC signal. When the ground electrode, typically the back surface electrode, becomes low impedance enough, this phenomenon does not appear. However, the EA-DFB device is installed within the package, the ground line can not widen and shorten enough due to less space within the package, which makes the ground line the common impedance to the EA portion and the DFB portion. Therefore, the high frequency signal applied to the EA portion appears in deformed, so the optical output because the EA portion is driven by this deformed signal, which increases the jitter and the noise involved in the optical output. For the DFB portion, the oscillation spectrum thereof has widened because, in spite of being drive by the DC signal, the high frequency signal leaks from the EA portion through the common impedance, which causes the chirp on the oscillation wavelength. These phenomena strongly affect the optical transmission characteristic and become one of primary reason to suppress the transmission distance.
Moreover, in products intended to operate in the high speed, how the dynamic testing can be performed dominates not only the performance of the products but also the cost thereof. When the dynamic testing is carried out after the completion of the product, the reliability of the testing may be secured. However, once deciding the failure for the product, all parts must be scraped although the failure is attributed to only a portion of parts. On the other hand, when the dynamic testing is carried out during the production, the reliability of the testing itself may not be secured due to the difference between the final and the intermediate configurations.
Furthermore, for the semiconductor light-emitting device and the semiconductor optical modulating device, a burn-in testing, typically called as the screening test, is often processed to remove failure products that may break in a short time. Operating under a predetermined condition, such as 85° C. for 48 hours, and comparing the characteristic of the device such as the threshold current, the emission efficiency, and the modulation performance, products showing relative large change for such characteristics before and after the operation should be dropped from the production. Thus, not only enhancing the reliability of the final products, but also the total production cost may be reduced. From this viewpoint, the configuration is necessary to enable the testing as the light-emitting device is mounted on the carrier.